Stretched multilayer cation exchange membranes

Abstract

A cation exchange membrane includes a stretched film including at least two layers of fluorinated ionomer containing sulfonate or sulfonic acid groups. The layers have differing ion exchange ratio values, which define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers. The high and low ion exchange ratio layers differ in ion exchange ratio by at least about 1. A process for making a cation exchange membrane includes forming a film including at least two layers of fluorinated ionomer containing sulfonate or sulfonic acid groups to form a multi-layer film and stretching the multi-layer film. An electrochemical cell has anode and cathode compartments and includes a cation exchange membrane as a separator between said anode and cathode compartments, where the membrane includes a stretched film including at least two layers of fluorinated ionomer containing sulfonate or sulfonic acid groups.

1 . A cation exchange membrane comprising a stretched film comprising at least two layers of fluorinated ionomer containing sulfonate or sulfonic acid groups, said layers having differing ion exchange ratio values which define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers, said high and low ion exchange ratio layers differing in ion exchange ratio by at least about 1. 
     
     
         2 . The membrane of  claim 1 , wherein the stretched film has an apparent ion exchange ratio in the range of about 7.1 to about 25.2. 
     
     
         3 . The membrane of  claim 2 , wherein the apparent ion exchange ratio is in the range of about 9.1 to about 23.2. 
     
     
         4 . The membrane of any of  claims 1-3 , wherein said high and low ion exchange ratio layers of fluorinated ionomer containing sulfonate or sulfonic acid groups differing in ion exchange ratio by at least at least about 2. 
     
     
         5 . The membrane of  claim 1 , where the one or more low ion exchange ratio layer has an ion exchange ratio of at most about 17.2. 
     
     
         6 . The membrane of  claim 1 , where the one or more high ion exchange ratio layers has an ion exchange ratio of at least about 11.1. 
     
     
         7 . The membrane of  claim 1 , where the stretched film is biaxially stretched. 
     
     
         8 . The membrane of  claim 1 , where the stretched film comprises at least two high ion exchange ratio layers. 
     
     
         9 . The membrane of  claim 8 , where the at least two high ion exchange ratio layers have equivalent ion exchange ratios. 
     
     
         10 . The membrane of  claim 1 , where the thickness of the one or more low ion exchange ratio layers is greater than or equal to the thickness of the one or more high ion exchange ratio layers. 
     
     
         11 . The membrane of  claim 1  having a membrane thickness after stretching in the range of about 10 μm to about 200 μm. 
     
     
         12 . The membrane of  claim 1 , wherein said membrane is unreinforced. 
     
     
         13 . A process for making a cation exchange membrane comprising a stretched film, said process comprising:
 forming a film comprising at least two layers of fluorinated ionomer containing sulfonate or sulfonic acid groups to form a multi-layer film; and   stretching the multi-layer film;   wherein said layers have differing ion exchange ratio values which define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers, said high and low ion exchange ratio layers differing in ion exchange ratio by at least about 1.   
     
     
         14 . The process of  claim 13 , further comprising the step of heating the multi-layer film to a temperature not lower than 20° C. below the glass transition temperature of the fluorinated ionomer. 
     
     
         15 . The process of  claim 13 or 14 , where the stretching is carried out at a rate of about 1%/sec to about 200%/sec. 
     
     
         16 . The process of  claim 13 , wherein the apparent ion exchange ratio of the layers of fluorinated ionomer containing sulfonate or sulfonic acid groups is about 7.1 to about 25.2. 
     
     
         17 . An electrochemical cell having anode and cathode compartments and comprising a cation exchange membrane as a separator between said anode and cathode compartments, said membrane comprising a stretched film comprising at least two layers of fluorinated ionomer containing sulfonate or sulfonic acid groups, said layers having differing ion exchange ratio values which define one or more high ion exchange ratio layers and one or more low ion exchange ratio layers, said high and low ion exchange ratio layers differing in ion exchange ratio by at least about 1. 
     
     
         18 . The electrochemical cell of  claim 17 , where the electrochemical cell is selected from the group consisting of a redox flow battery cell, a fuel cell, a chloralkali cell, and a water electrolysis cell. 
     
     
         19 . The electrochemical cell of  claim 18 , wherein said electrochemical cell is a redox flow battery cell. 
     
     
         20 . The electrochemical cell of  claim 19 , wherein said redox flow battery cell is a vanadium redox flow battery cell.